Electrosynthesis organics

In this chapter we shall consider the role of electrolysis in the manufacture of organic compounds. 

It will be shown below that many factors can be important in the assessment of organic electrosyntheses but they will commonly include  

The material yield of the desired product this is particularly important when the starting material is expensive. 

The type and quantity of by-products-by-products always increase the cost of isolation of pure product but the presence of trace impurities may completely rule out the use of the product in the application envisaged, e.g. very low levels of impurities can be critical in pharmaceutical preparations. 

The cost of isolating pure product from the electrolysis medium. Process strategies which reduce the number of unit processes in the product isolation stage can be very advantageous to the economics of an electrolytic process, 

On the other hand, what are the problems which presently prevent the widespread commercial exploitation of organic electrosynthesis First we must recognize that organic electrosynthetic processes are chemically much more complex than any other processes considered in this book. Usually the overall electrode reaction is not simple electron transfer, but is a sequence of electron transfers and coupled chemical processes either on the electrode surface or in 

Furthermore, the combination of electrochemistry and organic chemistry will almost always require compromise since the former is best suited to water containing high concentrations of electrolyte while the latter generally are better carried out in an organic medium. 

Organic electrosynthetic processes have also suffered from a failure to develop the cell components and the technology essential for successful operation. The reason is economic. One or even a few processes on a small or medium scale does not justify the development of optimum electrodes, membranes, etc., and in any case organic processes require the use of diverse media and conditions. As a result organic processes have often been constructed with inappropriate, although the best available, cell components and hence have been run with technology far from the optimum and this has undoubtedly affected their performance and reputation. 

In this chapter we shall consider first the largest-scale industrial process, the Monsanto hydrodimerization of acrylonitrile to adiponitrile, and then go on to discuss the other processes presently used or likely to be introduced in the near future. 

In particular, the concepts of potential control and the large driving force for chemical change available at electrodes generated two types of investigation. The first type concerned the realization that the first step in many electrode reactions is a simple, reversible one-electron (le ) transfer to/from the organic molecules and that stable intermediates -for example, anion radicals and cation radicals of aromatic compounds and transition 

Developments in Electrochemistry Science Inspired by Martin Fleischmann, First Edition. Edited by Derek Fletcher, Zhong-Qun Xian and David E. Williams. 

A typical electrolysis [4,5] involves the oxidation of cyclohexane to give methyl 2-methylcyclopent-l-enyl ketone in a single step via a mechanism involving carbenium ion intermediates and the chemistry of the acetyl cation. 

The yield of naphthaquinone was 90%. This process was initially developed in the laboratories of W.R. Grace and Company, but then scaled-up to an industrial unit by HydroQuebec and a Canadian company using commercially available cells, membranes, and electrode materials. 

The academic world has investigated more sophisticated mediators with a view to increasing the selectivity and sophistication of the catalyzed chemistry. Typical examples are  

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D. Degner ia E. Steckhan, ed.. Organic Electrosynthesis in Industry, Electrochemistry III, Topics in Current Chemisty, Vol. 148, Spriager-Vedag, Berlin, 1988,... 

Carbon electrodes are widely used in electrochemistry both in the laboratory and on the industrial scale. The latter includes production of aluminium, fluorine, and chlorine, organic electrosynthesis, electrochemical power sources, etc. Besides the use of graphite (carbons) as a virtually inert electode material, the electrochemical intercalation deserves special attention. This topic will be treated in the next paragraph. 

Cognet P, Wilhem AM, Delmas H et al (2000) Ultrasound in organic electrosynthesis. Ultrason Sonochem 7 163-167... 

Isochem has set up a multipurpose electrochemical unit devoted to organic electrosynthesis. The unit located in Pithiviers (France) [115] is dimensioned for the production of 80 tons a year of cystein derivatives, such as cysteine base salts, carbocysteine, acetylcysteine, and thiazolidin carboxylic acids. 

Baizer MM, Lund H (eds) (1991) Organic electrosynthesis, 3rd ed, Marcel Dekker, New York... 

Pletcher D (1993) Progress in organic electrosynthesis, Electrochem Processing, Innovations and Progress, April 21-23, Glasgow... 

A synthetically very potent and unique feature of organic electrosynthesis is the oxidative or reductive Umpolung of reactivity. Reactive acceptors are anodically available as radical cations in a wide variety by the oxidative Umpolung of donors. This way two donors can be coupled in one step if one of them is converted to an acceptor at the electrode. Chemically, at least two additional... 

Today, a large number of important technologies are based on or related to electrodes reactions. Besides the chlor-alkali and aluminium industries, energy conversion in batteries and fuel cells, electrodeposition, electrorefining, organic electrosynthesis, industrial and biomedical sensors, corrosion and corrosion protection, etc. are amogst those technologies. In many of them, kinetic, catalytic or specificity aspects of electrode processes are of enormous importance. 

As discussed in Sects. 3.4 and 4.5, electrode processes coupled with homogeneous chemical reactions are very frequent and their study is of interest in many applied fields, such as organic electrosynthesis, ecotoxicity, biosciences, environmental studies, among others [15-17]. In this section, multipulse techniques (with a special focus on Cyclic Voltammetry) are applied to the study of the reaction kinetics and mechanisms of electrogenerated species. 

The following experimental variables are of importance for the outcome of an organic electrosynthesis ... 

Lead has been much used as a cathode material in organic electrosynthesis 92 it has a high hydrogen overvoltage and is easy to work mechanically. However, it does not appear to be as useful as vitreous carbon or platinum for general voltammetric work. 

Griesbach U, Zollinger D, Putter H, Comninellis C (2005) Evaluation of boron doped diamond electrodes for organic electrosynthesis on a preparative scale. J Appl Electrochem 35 1265-1270... 

Degner D (1988) Organic electrosynthesis in industry. Top Curr Chem 148 1-95... 

Little RD, Moeller KD (2002) Organic electrosynthesis as a tool for synthesis. Electrochem Soc Interface 11 36-42... 

The industrial organic electrosynthesis reaction of greatest impact must be the Monsanto process for the hydrodimerization of acrylonitrile to... 

Since classification schemes tend to be rather boring, a few words are necessary to explain why this Section has been included. In the first place, the philosophy behind the synthetic use of organic redox processes in which electrons are transferred stoichiometrically is entirely different from that used in conventional synthesis. There is a risk—and anyone who has dealt with the thankless task of propagating the virtues of organic electrosynthesis can attest to it—that a redox reaction is only considered to be a way of transforming diverse functional groups into each other. This is not so ... 

Barhdadi, R., Courtinard, C., Nedelec, J.-Y. and Troupel, M. (2003) Room-temperature ionic liquids as new solvents for organic electrosynthesis. The first examples of direct or nickel-catalyzed electroreductive coupling involving organic halides. Chem. Commun. (Cambridge) 12, 1434-1435. 

It is recommended that organic electrosynthesis be carried out at a constant current at first, since the setup and operation are simple. Then the product selectivity and yield can be improved by changing current density and the amoimt of electricity passed [current (A) x time (i) = electricity (C)]. However, the electrode potential changes with the consumption of the starting substrate (more positive in case of oxidation or more negative in case of reduction). Therefore the product selectivity and current efficiency sometimes decrease, particularly at the late stage of electrolysis. 

Of course an appropriate ionic conductivity in the active layer is important as well. Organic solid polymer electrolytes (SPEs) provide only k = 0.1-1 mS/cm, and liquid organic electrolytes k — 1-lOmS/cm. But aqueous electrolytes have much better values in the order of lO-lOOmS/cm. One of the consequences of the low ionic conductivities of organic electrolytes is a minimization of transport length L (cf. Vetter s model [64]), or the capillary gap cell in organic electrosynthesis [4]. 

S Torii, T Hase, M Kuroboshi, T Katagiri, C Amatore, A Jutand, H Kawafuchi. Novel Trends in Organic Electrosynthesis. S Torii, ed. New York Springer-Verlag, 1998, p 87. 

D. Fletcher discusses these problems in his lectures, The Fundamentals of Electrosynthesis and Its Scale-Up [120] and Selling Electrochemical Technology Why, How, When [121]. In Ref. 66, he illustrates his experience of many years with factors important in the economics of organic electrosynthesis processes operated on various scales. Figure 20 reflects this outline from an industrial point of view. 

There is a trend toward chiral synthesis [132] the Englehard company is said to have piloted the electrosynthesis of chiral diols. New reactor design for the epoxidation of olefins is under development on a pilot scale [133]. Gas diffusion electrodes, developed for fuel cells and inorganic processes, are finding their first applications in organic electrosynthesis [134—136]. Another area of more than laboratory interest is bioelectrochemistry [137] (see also E. Steckhan, Chapter 27 in this volume). 

A Theoret. Challenge and Commercial Opportunities in Organic Electrosynthesis The Case of Anthraquinone, Ninth International Forum on Electrolysis. Clearwater, FL Electrosynthesis Company, E. Amherst, NJ 5-9 November 1995. 

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